Biomass and Remote Sensing of Biomass Part 8 pot

The data collected by each satellite sensor can be described in terms of spatial, spectral and temporal resolution.. Spatial resolution may also be described as the ground surface area t

their own source of energy; an example would be a radar gun These sensors send out a signal and measure the amount reflected back Active sensors are more controlled because they do not depend upon varying illumination conditions

Passive sensors Active sensors

Fig 2 Active and passive sensors

1.3.1 Orbits and swaths

The path followed by a satellite is referred to as its orbit Satellites which view the same

portion of the earth’s surface at all times have geostationary orbits Weather and

communication satellites commonly have these types of orbits Many satellites are designed

to follow a north south orbit which, in conjunction with the earth’s rotation (west-east), allows them to cover most of the earth’s surface over a period of time These are Near-polar orbits Many of these satellites orbits are also Sun-synchronous such that they cover each area of the world at a constant local time of day Near polar orbits also means that the satellite travels northward on one side of the earth and the southward on the second half of its orbit These are called Ascending and Descending passes As a satellite revolves around the earth, the sensor sees a certain portion of the earth’s surface The area imaged is referred

to as the Swath The surface directly below the satellite is called the Nadir point Steerable sensors on satellites can view an area (off nadir) before and after the orbits passes over a target

1.3.1.1 Satellite sensor characteristics

The basic functions of most satellite sensors are to collect information about the reflected radiation along a pathway, also known as the field of view (FOV), as the satellite orbits the Earth The smallest area of ground that is sampled is called the instantaneous field of view (IFOV) The IFOV is also described as the pixel size of the sensor This sampling or measurement occurs in one or many spectral bands of the EM spectrum The data collected

by each satellite sensor can be described in terms of spatial, spectral and temporal resolution

1.3.1.2 Spatial resolution

The spatial resolution (also known as ground resolution) is the ground area imaged for the instantaneous field of view (IFOV) of the sensing device Spatial resolution may also be described as the ground surface area that forms one pixel in the satellite image The IFOV or

ground resolution of the Landsat Thematic Mapper (TM) sensor, for example, is 30 m The ground resolution of weather satellite sensors is often larger than a square kilometre There are satellites that collect data at less than one meter ground resolution but these are classified military satellites or very expensive commercial systems

1.3.1.3 Temporal resolution

Temporal resolution is a measure of the repeat cycle or frequency with which a sensor revisits the same part of the Earth’s surf ace The frequency will vary from several times per day, for a typical weather satellite, to 8—20 times a year for a moderate ground resolution satellite, such as Landsat TM The frequency characteristics will be determined by the design

of the satellite sensor and its orbit pattern

1.3.1.4 Spectral resolution

The spectral resolution of a sensor system is the number and width of spectral bands in the sensing device The simplest form of spectral resolution is a sensor with one band only, which senses visible light An image from this sensor would be similar in appearance to a black and white photograph from an aircraft A sensor with three spectral bands in the visible region of the EM spectrum would collect similar information to that of the human vision system The Landsat TM sensor has seven spectral bands located in the visible and near to mid infrared parts of the spectrum

A panchromatic image consists of only one band It is usually displayed as a grey scale

image, i.e the displayed brightness of a particular pixel is proportional to the pixel digital number which is related to the intensity of solar radiation reflected by the targets in the

Fig 3 Electromagnetic radiation spectrum with different resolution bands

pixel and detected by the detector Thus, a panchromatic image may be similarly interpreted

as a black-and-white aerial photograph of the area, though at a lower resolution

Multispectral and hyperspectral images consist of several bands of data For visual display, each band of the image may be displayed one band at a time as a grey scale image, or in combination of three bands at a time as a color composite image Interpretation of a multispectral color composite image will require the knowledge of the spectral reflectance signature of the targets in the scene

1.3.2 Platforms

Aerial photography has been used in agricultural and natural resource management for many years These photographs can be black and white, colour, or colour infrared Depending on the camera, lens, and flying height these images can have a variety of scales Photographs can be used to determine spatial arrangement of fields, irrigation ditches, roads, and other features or they can be used to view individual features within a field

Infrared images can detect stress in crops before it is visible with the naked eye Healthy

canopies reflect strongly in the infrared spectral range, whereas plants that are stressed will reflect a dull colour These images can tell a farmer that there is a problem but does not tell him what is causing the problem The stress might be from lack of water, insect damage, improper nutrition or soil problems, such as compaction, salinity or inefficient drainage The farmer must assess the cause of the stress from other information If the dull areas disappear

on subsequent pictures, the stress could have been lack of water that was eased with irrigation If the stress continues it could be a sign of insect infestation The farmer still has

to conduct in-field assessment to identify the causes of the problem The development of cameras that measure reflectance in a wider range of wavelengths may lead to better quantify plant stress The uses of these multi-spectral cameras are increasing and will become an important tool in precision agriculture

Satellite remote sensing is becoming more readily available for use in precision agriculture

The Landsat and the NOAA polar-orbiting satellites carry instruments that can be used to

determine crop types and conditions, and to measure crop acreage The Advanced Very High Resolution Radiometer (AVHRR) carried onboard NOAA polar orbiting satellites measure reflectance from the earth’s surface in the visible, near infrared, and thermal infrared portions of the electromagnetic spectrum

This spectral sensitivity makes it suitable for measuring vegetative condition and because the satellite passes overhead twice a day, it can be used to detect rapidly changing conditions Unfortunately, its use as a precision agriculture tool is limited because the spatial resolution of the sensor is nominally 1.1km A possible application of this scanner would be to use the thermal infrared sensor to estimate daily maximum and minimum temperatures These temperature estimates could then be used to determine degree-days that will drive pest development models

Degree-day models are an essential part of IPM programs and the enhanced spatial coverage provided by satellites would allow for assessment of spatial variability in predicted events that is not possible with data from sparsely spaced weather stations currently used for these models Remotely sensed data can also be used to determine irrigation scheduling and adequacy of irrigation systems for uniformly wetting an entire field The sensors aboard the Landsat satellite measures reflected radiation in seven

spectral bands from the visible through the thermal infrared The sensors high spatial resolution (approximately 30m) makes it useful in precision agriculture The spectral response and higher spatial resolution make it suitable for assessing vegetative condition for individual fields but the overpass frequency is only once every 16 days The less frequent overpass makes it difficult to use these data for assessing rapidly changing events such as insect outbreaks or water stress New satellites with enhanced capabilities are planned and remotely sensed data will become more widely used in management support systems

Fig 4 Figure Advanced Very High Resolution Radiometer (AVHRR) image of the

southwest Untied States Image is centred on the Las Cruces, New Mexico

1.3.3 Common satellites

1.3.3.1 GOES

5 spectral bands 1 - 41 km spatial resolution Geostationary

1.3.3.2 NOAA AVHRR

5 spectral bands 1.1 km spatial resolution 1 day repeat cycle

1.3.3.3 Landsat TM

7 spectral bands 30m spatial resolution 16 day repeat cycle

1.3.3.4 MODIS

Multi- spectral bands 250-1000m spatial resolution (band dependent) 1day repeat cycle

1.3.3.5 IKONOS

4 spectral Bands 4m spatial resolution 5 day repeat cycle

1.4 Spectral signatures of natural and human-made materials

Remote sensing makes use of visible, near infrared and short-wave infrared sensors to form images of the earth's surface by detecting the solar radiation reflected from targets on the ground Different materials reflect and absorb differently at different wavelengths Thus, the targets can be differentiated by their spectral reflectance signatures in the remotely sensed images

Fig 5 Refraction and diffraction of radiations by different objects

1.5 Spectral reflectance signature

When solar radiation hits a target surface, it may be transmitted, absorbed or reflected Different materials reflect and absorb differently at different wavelengths The reflectance spectrum of a material is a plot of the fraction of radiation reflected as a fun tion of the incident wavelength and serves as a unique signature for the material In principle, a material can be identified from its spectral reflectance signature if the sensing system has

sufficient spectral resolution to distinguish its spectrum from those of other materials This

premise provides the basis for multispectral remote sensing The following graph shows the typical reflectance spectra of water, bare soil and two types of vegetation

Fig 6 Spectral resolution of different materials

The reflectance of clear water is generally low However, the reflectance is maximum at the blue end of the spectrum and decreases as wavelength increases Hence, water appears dark bluish to the visible eye Turbid water has some sediment suspension that increases the reflectance in the red end of the spectrum and would be brownish in appearance The reflectance of bare soil generally depends on its composition In the example shown, the reflectance increases monotonically with increasing wavelength Hence, it should appear yellowish-red to the eye

Vegetation has a unique spectral signature that enables it to be distinguished readily from

other types of land cover in an optical/near-infrared image The reflectance is low in both the blue and red regions of the spectrum, due to absorption by chlorophyll for photosynthesis It has a peak at the green region In the near infrared (NIR) region, the reflectance is much higher than that in the visible band due to the cellular structure in the leaves Hence, vegetation can be identified by the high NIR but generally low visible reflectance This property has been used in early reconnaissance missions during war times for "camouflage detection"

The shape of the reflectance spectrum can be used for identification of vegetation type For example, the reflectance spectra of dry grass and green grass in the previous figures can be distinguished although they exhibit the generally characteristics of high NIR but low visible reflectance Dry grass has higher reflectance in the visible region but lower reflectance in the NIR region For the same vegetation type, the reflectance spectrum also depends on other factors such as the leaf moisture content and health of the plants These properties enable vegetation condition to be monitored using remotely sensed images

Fig 7 Reflectance spectrum of different materials

1.6 Geodesy, geodetic datums and map projections

Geodesy is the branch of science concerned with the determination of the size and shape of

the Earth Geodesy involves the processing of survey measurements on the curved surface

of the Earth, as well as the analysis of gravity measurements Knowing the exact location of

a pixel on the Earth’s surface (its spatial location) is an essential component of remote sensing It requires a detailed knowledge of the size and the shape of the Earth The Earth is

not a simple sphere Topographic features such as mountain ranges and deep oceans disturb the surface of the Earth The ideal reference model for the Earth’s shape is one that can represent these irregularities and identify the position of features through a co-ordinate system It should also be easy to use

1.6.1 Flat Earth vs curved Earth

The “flat Earth” model is not appropriate when mapping larger areas It does not take into account the curvature of the Earth A “curved Earth” model more closely represents the shape of the Earth A spheroid best represents the shape of the Earth because it is significantly wider at the equator than around the poles (Unlike a simple sphere) A spheroid, (also known as an ellipsoid) represents the equator as an elliptical shape, rather than a round circle Surveying and navigation calculations can he performed over a large area when a spheroid is used as a curved Earth reference model

1.6.2 Sea level and the composition of the Earth’s interior

The surface of the sea is not uniform The Earth’s gravitational field shapes it The rocks that make up the Earth’s interior vary in density and distribution, causing anomalies in the gravitational field These, in turn, cause irregularities in the sea surface A mathematical model of the sea surface can be formulated; however, it is very complex and not useful for finding geographic positions on a spheroid reference model

1.6.3 Types of geodetic datum

Based on these ideas, models can be established from which spatial position can be calculated These models are known as geodetic datums and are normally classified into two types geocentric datum and local geodetic datum

A geocentric datum is one which best approximates the size and shape of the Earth as a

whole The center of its spheroid coincides with the Earth’s center of mass A geocentric

datum does not seek to be a good approximation to any particular part of the Earth A local

geodetic datum is used to approximate the size and shape of the Earth’s sea surface in a smaller area

Datums and GIS

Having a standard accurate datum set becomes increasingly important as multiple layers of information about the same area are collected and analyzed The layers are developed into geographic information systems (GIS), which enable the relationships between layers of data to be examined In order to function effectively, a GIS must possess one essential attribute It must have the ability to geographically relate data within and across layers For example, if a dataset about vegetation is being examined against the data sets for topography and soils, the accurate spatial compatibility of the two datasets is critical

1.6.4 Map projection coordinates

A map projection is a systematic representation of all or part of the Earth on a two dimensional surface, such as a flat sheet of paper During this process some distortion of distances, directions, scale, and area is inevitable There are several different types of map

projections No projection is free from all distortions, but each minimizes distortions in some

of the above properties, at the expense of leaving errors in others For example, the commonly used Transverse Mercator projection represents direction accurately, but distorts distance and area, especially those farthest from the equator Greenland, for example, appears to be much larger than it really is The Transverse Mercator projection is useful for navigation charts

1.6.4.1 Universal Transverse Mercator (UTM)

Universal Transverse Mercator (UTM) is a global spatial system based on the Transverse Mercator projection UTM divides the Earth into 60 equal zones, each being 6 degrees wide Each zone is bounded by lines of longitude extending from the North Pole to the South Pole Imagine an orange consisting of 60 segments Each segment would be equivalent to a UTM zone A rectangular grid coordinate system is used in most map projections These coordinates are referred to as Eastings and Northings, being distances East and North of an origin They are usually expressed in metres Under the UTM system, each East and North coordinate pair could refer to one of sixty points on Earth — one point in each of the sixty zones Because of this, the zone number needs to be quoted to ensure the correct point on Earth is being identified

1.6.4.2 Global Positioning System

The Global Positioning System (GPS) is a satellite based system that gives real time three dimensional (3D) latitude, longitude, and height information at sub-meter accuracy The system was developed by the United States military in the late 1970’s to give troops accurate position and navigational information A GPS receiver calculates its position on earth from radio signals broadcast by satellites orbiting the earth There are currently twenty-four GPS satellites in this system GPS equipment is capable of measuring a position to within centimetres but the accuracy suffers due to errors in the satellite signals Errors in the signal can be caused by atmospheric interference, proximity of mountains, trees, or tall buildings The government can also introduce errors in the signal for security purposes This intentional degradation of the satellite signals is known as selective availability The accuracy of the position information can be improved by using differential GPS In differential GPS, one receiver is mounted in a stationary position, usually at the farm office, while the other is on the tractor or harvesting equipment The stationary receiver calculates the error and transmits the necessary correction to the mobile receiver GPS equipment suitable for precision agricultural cost several thousand dollars Less expensive equipment is becoming available but the accuracy and capability

is reduced

1.6.4.3 Geographic Information System (GIS)

A Geographic Information System (GIS) is a computer-assisted system for handling spatial

information GIS software can be considered as a collection of software programs to acquire, store, analyze, and display information The input data can be maps, charts, spreadsheets, or pictures The GIS software can analyze these data using image processing and statistical procedures Data can be grouped together and displayed as overlays Overlays could be information such as soil type, topography, crop type, crop yield, pest levels, irrigation, and management information as shown

Fig 8 Topographic GIS map of the forest area

Relationships can be examined and new data sets produced by combining a number of overlays These data sets can be combined with models and decision support systems to construct a powerful management tool For example, we could assess how far a field was from roads or non-agricultural crops This information could be important in pest infestation or in planning chemical application We could also examine crop yield relationship to soil type or other factors as show in the following figure A number of GIS software packages are now commercially available Spatial data for the GIS is often collected using GPS equipment but another source of spatial information is aerial and satellite imagery

1.6.4.4 Pixels, images and colours

1.6.4.4.1 Colour composite images

In displaying a colour composite image, three primary colors (red, green and blue) are used when these three colours are combined in various proportions, they produce different colors

in the visible spectrum Associating each spectral band (not necessarily a visible band) to a separate primary colour results in a colour composite image

Fig 9 Primary colour composite

Many colours can be formed by combining the three primary colours (Red, Green, Blue) in

various proportions

1.6.4.4.2 False colour composite

The display colour assignment for any band of a multispectral image can be done in an

entirely arbitrary manner In this case, the colour of a target in the displayed image does not

have any resemblance to its actual colour The resulting product is known as a false color composite image There are many possible schemes of producing false color composite

images However, some scheme may be more suitable for detecting certain objects in the image

1.6.4.4.3 Natural colour composite

When displaying a natural colour composite image, the spectral bands (some of which may not be in the visible region) are combined in such a way that the appearance of the displayed image resembles a visible colour photograph, i.e vegetation in green, water in

blue, soil in brown or grey, etc Many people refer to this composite as a "true colour"

composite However, this term may be misleading since in many instances the colours are only simulated to look similar to the "true" colours of the targets For example, the bands 3

(red band), 2 (green band) and 1 (blue band) of a AVHRR image can be assigned

respectively to the R, G, and B colours for display In this way, the colour of the resulting colour composite image resembles closely what the human eyes would observe

Fig 10 Development of natural colour composite

1.7 Image processing and analysis

Many image processing and analysis techniques have been developed to aid the interpretation of remote sensing images and to extract as much information as possible from the images The choice of specific techniques or algorithms to use depends on the goals of

each individual project The key steps in processing remotely sensed data are Digitizing of Images, Image Calibration, Geo-Registration, and Spectral Analysis Prior to data analysis,

initial processing on the raw data is usually carried out to correct for any distortion due to the characteristics of the imaging system and imaging conditions Depending on the user's requirement, some standard correction procedures may be carried out by the ground station